Marine vessels use various propulsion systems or units. The main propulsion unit or units is/are normally arranged in the aft part of the ship. The main propulsion unit may be either a fixed propeller arrangement creating a thrust force in the longitudinal direction of the marine vessel, or it may be a pod or a thruster, i.e. a propeller arrangement that may be rotated round a vertical axis.
Normally the pods or thrusters are initially installed from beneath in an opening, so called well, at the hull of the marine vessel when the marine vessel is dry docked, i.e. not yet afloat in water. However, when the pod or thruster needs maintenance or needs to be replaced or modernized, the marine vessel is normally not dry docked but is afloat in water and the thruster is removed by a specific hoisting method and arrangement, which is also used when installing a new or repaired thruster back into the well.
The well at the hull of a prior art marine vessel is provided with a well box, which is welded of its bottom flange or by some other appropriate water-tight manner secured to the bottom of the marine vessel. The well box forms within the hull of a marine vessel a water-tight cavity into which the parts of the thruster located within the hull are fitted. The well box comprises an annular wall with top and bottom flanges, a top cover that is bolted on the top flange of the well box to separate the well box interior from the interior of the hull in an air/water-tight manner for the time of installation of the thruster until the sealing of the thruster keep the water away from the interior of the well box, and three hoisting pipes that are located radially outside the annular wall, on different circumferential positions (for instance spaced 120 degrees from one another) and extend vertically from the well box upwards. The well box comprises further a bottom cover that is bolted to the bottom of the well box to close the well box interior from water. The hoisting pipes are also provided with similar covers outside the perimeter of the bottom cover of the well box.
When a prior art thruster is about to be mounted in the well box, a diver is needed for performing a number of tasks. Firstly, the diver is needed for demounting the covers of the hoisting pipes and for connecting the covers to ropes to avoid the covers from sinking. Next, three hoisting cables are lowered through the hoisting pipes in the water for the diver to attach the cables to brackets of the bottom cover, whereafter the safety bolts fastening the bottom cover may be demounted. Thereafter pressurized air is applied in the interior of the well box to push the bottom cover out. Then, a crane lowers the bottom cover by means of the hoisting cables and a lifting cable from outside the marine vessel is connected by the diver to a single lifting point of the bottom cover. The single lifting cable pulls the bottom cover such that the cable is vertical from the crane, whereafter the hoisting cables that run through the hoisting pipes are disconnected from the bottom cover and the bottom cover is hoisted onto the deck of the vessel by means of the single lifting cable.
Next the prior art thruster to be mounted is coupled to the lifting cable and lowered to water by the crane operating the lifting cable. The three hoisting cables are connected to the thruster, the thruster is drawn by the crane operating the hoisting cables below the hull of the ship and the lifting cable is disconnected by the diver. Thereafter the thruster is lifted by the hoisting cables under the supervision of the diver towards the well box. Simultaneously, the diver checks that the well box is filled with air, and if not instructs the operator to apply pressurized air into the interior of the well box. Thereafter, the diver makes sure the thruster is hanging with its mounting flange horizontally, whereafter the thruster may be lifted into the well box, while the diver monitors that the hoisting cables run properly in the hoisting pipes, the thruster enters the well box correctly and that guide pins in the mounting flange of the thruster enter properly their respective guide openings in the mounting flange of the well box. Next, the driver makes sure the mounting flanges are parallel and, if not, he instructs the operator of the crane to lift or to lower one or two hoisting cables to result in parallel positioning of the mounting flanges. Thereafter, the thruster may be lifted until the mounting flanges meet, and safety bolts may be inserted from inside the hull to secure the thruster against the well box. Thereafter the tightness of the thruster mounting is checked and, if no water is leaking into the well box, the final mounting studs are mounted and tightened to a predetermined tension from inside the hull. Simultaneously, the diver disconnects the hoisting cables from the thruster and mounts the covers of the hoisting pipes by means a number of bolts.
As may be seen from above the diver has a lot of demanding physical tasks to perform in addition to the responsibility of the outcome of the entire mounting operation.
Another weakness in the underwater mounting of a thruster is related to the mechanical construction of the well box, the thruster and their sealing arrangement. The present sealing arrangement of a thruster is formed of a coarse seal and an O-ring seal. Both seals are arranged between the radially extending surfaces of the mounting flanges of the well box and the thruster. The coarse seal is arranged into a groove having a longer radius than that arranged for the O-ring seal. Both grooves are normally in the mounting flange of the thruster. However, the prior art construction of both the coarse seal and the O-ring seal being at the lowermost level of the mounting flange means, in practice, that the coarse seal is used, tightened or compressed only by the above mentioned safety bolts, to confirm whether the sealing leaks or not. This is by no means a reliable way of determining the water tightness of the sealing as even the slightest misalignment between the mounting flange of the thruster and the bottom flange of the well box cause a minor leak even if there is nothing wrong with the actual sealing. Thus, as the water leakage means, in practice, that the thruster has to be lowered and the entire sealing arrangement has to be checked either in water or, at its worst, as lifted out of the water, the whole installation process of the thruster may be repeated without any true cause. In other words, the diver who is responsible for the accuracy of the entire installation should measure the distance between the two flanges and make sure there is not the slightest misalignment between the surfaces. It is not hard to imagine how difficult this task is taken that the waters where the servicing is normally are muddy and thereby far from clear, whereby the risk of not noticing a slight misalignment is great.
In view of the above an object of the present invention is to reduce the need for a diver when dismounting and mounting a thruster of an afloat marine vessel.
Another object of the present invention is to propose a sealing arrangement that is not sensitive to slight misalignment in the installation phase.
Yet another object of the present invention is to improve the sealing between the thruster and the well box.
A further object of the present invention is to propose a mounting flange—well box construction, where there are several guide surfaces ensuring the correct alignment between the mating surfaces of the mounting flange of the thruster and the bottom flange of the well box.